Conventional infrared emission sources typically utilize a wire, metal or ceramic element that is heated to emit a continuous band of optical radiation. In particular, the source elements, e.g., an infrared igniter source element, that is incorporated into infrared spectrometers, such as, FTIR instruments, are often modifications or adaptations of commercial heating elements. Examples include filaments configured from resistive electrical conductive materials, such as Kanthal, (i.e., alloys known for their ability to withstand high temperatures and having intermediate electric resistance) and those manufactured from silicon carbide. Another example includes the use of a silicon nitride-tipped glo-plug used for preheating the combustion chamber in a diesel engine.
Stabilization of the source, such as when using silicon carbide, however, can be a major issue, especially when the heating element is exposed to ambient conditions. For example, air turbulence in proximity to the source can cause localized cooling in addition to source noise induced from index of refraction of changes resulting from hot air close to the source and nearby cooler air. Typically, these issues can be reduced by surrounding the source element with a thermally insulating enclosure but with an opening in the enclosure to enable a beam of desired infrared radiation to exit in a designed manner. The insulation, in such an arrangement, helps stabilize the output in addition to enabling operating voltages to be decreased.
Another egregious instability, however, when using carbide elements as the igniter source, is that as the element is heated to temperatures approaching 1300° C., material changes, such as oxidation and thermal degradation, can produce inhomogeneities in the desired spectral output so as to affect the measurement results and thus necessitating, for example, artificial spectral corrections of the source.
Another source of spectral instability occurs in the imaging of the source itself due to its geometry. In operation, an igniter source element when made from highly resistive silicon carbide is often configured as a loop, i.e., a U-shaped heating element, which is electrically coupled to a power source so as to heat the element to a high temperature that enables the element to radiate over a broad range of predetermined infrared electromagnetic bandwidths. The issue arises in imaging the central portion of the U-shape because there is a resultant loss of emissivity in the central portion as opposed to the outer portions of the optically relayed image directed throughout the spectrometer, which can cause problems when interpreting the spectral information.
Background information on an apparatus that provides for such an infrared source, the disclosure of which is incorporated herein by reference in its entirety can be found in U.S. Pat. No. 5,291,022, to Drake et al., issued Mar. 1, 1994, entitled; “High Efficiency Infrared Source,” including the following: “An infrared source for use in an infrared spectrometer includes an insulator core having a containment cavity, an outlet port in communication with the containment cavity, and an electrically heated infrared element mounted in the containment cavity with a portion thereof facing the outlet port and with the walls of the containment cavity closely spaced to the infrared element. The insulator core is formed of a ceramic fiber material which has excellent resistance to heat and very low thermal conductivity so that very little heat from the infrared element escapes from the insulator core except as infrared radiation through the outlet port. The insulator core is preferably mounted within a central cavity of a metal housing, and may be sealed off from the ambient atmosphere by an infrared transmissive window sealed to an outlet opening in the housing. The electrical supply lines from the infrared element may extend through an opening in the housing which is closed and sealed to inhibit the passage of gases from the ambient atmosphere into the interior of the housing. Where the infrared element is sealed off from the ambient atmosphere in this manner, potentially corrosive gases will be inhibited from reaching the hot infrared element. This containment of the infrared element within the insulator core allows the element to be maintained at a desired temperature for radiating infrared for use in analytical instruments such as infrared spectrometers, while consuming very low amounts of electrical power.”
In general, conventional systems that utilize sources described, as discussed above, have had problems associated with imaging of the source as well as material changes in the infrared elements that cause changes in the emitted spectral output. Accordingly, a need exists for an improved infrared emissivity source that can address the desires of the technical community and thus, the present invention is directed to such a need.